1. Field of the Invention
The field of the invention is direct sequence spread spectrum systems, and more specifically, direct sequence spread spectrum systems in which traffic and forward error correction codes relating thereto are transmitted in parallel over separate channels.
2. Background
Wireless communication systems are an integral component of the ongoing technology revolution. Mobile radio communication systems, such as cellular telephone systems, are evolving at an exponential rate. In a cellular system, a coverage area is divided into a plurality of xe2x80x9ccellsxe2x80x9d. A cell is the coverage area of a base station or transmitter. Low power transmitters are utilized, so that frequencies used in one cell can also be used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a xe2x80x9ccellxe2x80x9d served by a base station.
Cellular networks provide mobile communications ability for wide areas of coverage. The networks essentially replace the traditional wired networks for users in large areas. But wireless technology can also be used to replace smaller portions of the traditional wired network.
Each home or office in the industrialized world is equipped with at least one phone line. Each line represents a connection to the larger telecommunications network. This final connection is termed the local loop and expenditures on this portion of the telephone network account for nearly half of total expenditures. Wireless technology can greatly reduce the cost of installing this portion of the network in remote rural areas historically lacking telephone service, in existing networks striving to keep up with demand and in emerging economies trying to develop their telecommunications infrastructure.
Another area in which wireless technology is aiding telecommunications is in the home where the traditional telephone handset is being replaced by the cordless phone system. A cordless phone system is in many ways a mini version of a WLL system. Cordless handsets in the home allow for untethered use of the handset enabling the user the freedom to move about as long as they stay in range of the base station.
Wireless systems can be classified according to the method used to provide access to multiple users seeking to utilize the system in parallel. In Frequency Division Multiple Access (FDMA) systems, the available frequency spectrum is divided into multiple narrow bands, each of which defines a separate channel. Different users are allocated different bands. Since the bands are separated by frequency, multiple users can access the system in parallel.
In Time Division Multiple Access Systems (TDMA), the available frequency spectrum is divided into multiple narrow bands, and each band is in turn divided into multiple time slots. A channel is defined as a particular time slot within one of the frequency bands. Again, since the channels are separated in time, or time and frequency as the case may be, multiple users can access the system in parallel.
In Code Division Multiple Access (CDMA) or Direct Sequence. Spread Spectrum (DSSS) systems, channels are defined by complementary, orthogonal or pseudo-random spreading sequences or codes. The spreading sequence has a frequency much higher than that of a user""s information signal. Each user is assigned a unique spreading sequence. At the transmitter, the user""s information signal is multiplied by the spreading sequence assigned to the user. Since the frequency of the spreading sequence is much higher than that of the user""s information signal, the bandwidth of the information signal is effectively spread by this action.
The spread signals for each of the users are then simultaneously or concurrently transmitted over the same wideband frequency spectrum. As the receiver, each user""s information signal is retrieved from the received signal by multiplying the received signal by the spreading sequence for the user, and then integrating and sampling the product. Since the spreading sequences are orthogonal or pseudo-random, each user""s information signal can be retrieved from the same received signal.
A block diagram of a transmitter 110 in a DSSS system is depicted in FIG. 7. A user""s analog information signal is input to A/D converter 111, which digitizes the signal into bits. A block or frame of bits is then input to multiplexor 112, which adds error detection or check bits, such as Cyclic Redundancy Check (CRC) bits, to the frame. The frame of bits, including the check bits, is then input to Forward Error Correction (FEC) coding block 113, which encodes group of bits into codewords using one of the many known FEC coding schemes, such as convolutional coding. Typically, a group of bits is translated into one codeword. Thus, the coder 113 typically introduces redundancy into the system. The FEC symbols are used at the receiver to perform error correction.
The symbols from the FEC coder 113 are then input to spreader 115. A unique spreading sequence for a user is generated by spreading code generator 114. The sequence, which comprises a series of chips, is input to the spreader 115. The spreader then spreads in frequency the codeword from FEC coder 113. Typically, the spreader performs this function by multiplying or XORing the symbols from coder 113 and the spreading sequence from generator 114.
Since the frequency of the spreading sequence is typically much greater than that of the information sequence, the effect of this process is to convert the information signal from a narrowband signal, depicted in FIG. 9 with identifying numeral 130, to a wideband signal, depicted in FIG. 9 with identifying numeral 131.
An advantage of a DSSS or CDMA system compared to narrowband systems, such as FDMA or TDMA, is its ability to withstand interference from a jamming signal. This property is illustrated in FIG. 9, which illustrates an interfering jammer 132. As can be seen, the effect on a narrowband signal 130 occupying the same or overlapping spectrum as the jammer is quite severe, whereas the effect on the wideband signal 131 is relatively minor.
The processing gain is a measure of the ability of a DSSS system to withstand interference from a jammer. Mathematically, it is given by W/RB, where W is the bandwidth of the spread signal, and RB is the bit rate of the incoming information signal. In the case in which the chip rate, Rc, of the spreading sequence is much greater than the bit rate RB, the processing gain is approximately equal to Rc/RB.
The coding gain of a system employing a particular form of error detection or correction coding is the amount (in dB) of reduction of Eb/No, the energy per bit divided by the noise density, that can be achieved at a given bit error rate (BER) by virtue of the coding. FIG. 15 illustrates the shift in the plot of Eb/No which results from implementing a certain error detection or correction code. The amount xcex94 (CG) of shift at a given BER is the coding gain.
In current DSSS systems, a problem is that there is a tradeoff between processing gain and coding gain such that some processing gain has to be sacrificed in order to achieve an improvement in coding gain, and vice-versa. The reason is that the achievement of coding gain requires the addition of redundancy into the system on the incoming information sequence, and the addition of this redundancy necessitates an increases in the incoming information bit rate, RB, in order to keep the throughput of the system the same. This increase in the information bit rate results in a decrease in the processing gain.
Another problem is that, due to limitations imposed by the frame structure, the class of FEC coding schemes which can be employed in such systems is limited.
Another problem with current DSSS systems is that there is a lack of robustness in responding to changing physical channel conditions. The reason is that the FEC coding scheme in such systems is relatively fixed and selected to deal with the worst case scenario. Therefore, when the performance of the physical channel exceeds the worst case scenario, the system is unable to respond with a less redundant FEC coding scheme. The result is that an excessive degree of redundancy is added to the system.
Moreover, efforts to dynamically respond to changing channel conditions with updated FEC coding schemes have been fraught with problems. In one such effort, a changing channel condition is detected, and, responsive thereto, an updated FEC coding scheme is selected and communicated to the receiver through a control channel in parallel with the traffic channel. The problem with this scheme is that there is no ability to respond to the changing channel condition if the control channel is busy or otherwise unavailable or becomes unreliable. Another problem is that the need to maintain a control channel decreases the number of traffic channels which can be maintained.
Accordingly, there is a need for a direct sequence spread spectrum system which overcomes the disadvantages of the prior art.
In accordance with the purpose of the invention as broadly described herein, there is provided a direct sequence spread spectrum wireless communication system comprising: a transmitter for (a) receiving a first block of data, (b) deriving therefrom a plurality of substantially redundant second blocks, (c) spreading each of the second blocks, each with a distinct spreading sequence, (d) deriving one or more signals representative of each of the second blocks, and (e) transmitting each of the one or more signals over a wireless interface; and a receiver for (a) receiving each of the one or more signals as transmitted over the wireless interface, (b) deriving therefrom estimates of one or more of the second blocks, and (c) deriving an estimate of the first block from one or more of the estimated second blocks. In one implementation, the second blocks is smaller than the first block. In other implementations, it can be larger than, or equal in size to the first block. In one implementation example, the blocks are frames.
There is also provided a transmitter configured for (a) receiving a first block of data, (b) deriving therefrom a plurality of substantially redundant second blocks, (c) spreading each of the second blocks, each with a distinct spreading sequence, (d) deriving therefrom one or more signals representative of each of the second blocks, and (e) transmitting each of the one or more signals over a wireless interface.
There is further provided a receiver configured for (a) receiving one or more signals representative of a plurality of substantially redundant second blocks over a wireless interface, (b) deriving therefrom estimates of one or more of the second blocks, and (c) deriving an estimate of a first block from one or more of the estimated second blocks.
A method for transmitting information over a wireless interface is also provided. In one embodiment, the method comprises the steps of: receiving a first block of data; deriving therefrom a plurality of substantially redundant second blocks; spreading each of the second blocks, each with a distinct spreading sequence; deriving therefrom one or more signals representative of each of the second blocks; transmitting each of the one or more signals over a wireless interface; receiving each of the one or more signals over the wireless interface; deriving therefrom estimates of one or more of the second blocks; and deriving an estimate of the first block from one or more of the estimated second blocks.
In a second embodiment, a method of transmitting information over a wireless interface comprises the following steps: receiving a first block of data; deriving therefrom a plurality of substantially redundant second blocks; spreading each of the second blocks, each with a distinct spreading sequence; deriving therefrom one or more signals representative of each of the second blocks; and transmitting each of the one or more signals over a wireless interface.
Also provided is a method of receiving information over a wireless interface comprising the following steps: receiving over a wireless interface one or more signals representative of a plurality of substantially redundant second blocks; deriving therefrom estimates of one or more of the second blocks; and deriving an estimate of a first block from one or more of the estimated second blocks.